1. Field of the Invention
This invention relates to a nitride semiconductor device structure, and more particularly to a nitride semiconductor device having the structure of a heterojunction field effect transistor based on a heterostructure.
2. Background Art
Circuits such as switching power supplies and inverters are based on power semiconductor devices including switching devices and diodes, which are required to have such characteristics as high withstand voltage and low on-resistance (RON). There is a tradeoff relation between the withstand voltage and the on-resistance (RON), which relation depends on the device material. With the progress of technology development, the on-resistance (RON) of power semiconductor devices is reduced to nearly the limit for silicon (Si), which has been the main device material. For further reduction of on-resistance (RON), the device material needs to be changed. For example, wide bandgap semiconductors such as gallium nitride (GaN), aluminum gallium nitride (AlGaN), and other nitride semiconductors and silicon carbide (SiC) can be used as switching device materials to improve the tradeoff relation determined by the device material, thereby dramatically reducing on-resistance (RON).
On the other hand, nitride semiconductors such as GaN and AlGaN can be used for heterojunction field effect transistors (HFETS) based on the AlGaN/GaN heterostructure. HFETs achieve low on-resistance through the high mobility of the heterointerface channel and the high electron concentration due to piezopolarization caused by heterointerface strain.
An HFET structure based on nitride semiconductors is disclosed (JP2001-168111A). In this structure, a source electrode, a gate electrode, and a drain electrode are formed on an n-type GaN channel layer, and a p-type GaN layer is formed under the n-type GaN channel layer to extract holes into the p-type GaN layer.